1. Field of the Invention
The present invention relates to a continuously variable transmission controller, and more particularly, to a continuously variable transmission controller suitable for reducing a shift shock generated when a drive mode is changed causing a shift characteristic to largely change.
2. Description of Background Art
A controller (See, for example, JP-A No. 123351/1994) for a belt-type continuously variable transmission connected to an internal-combustion engine (hereinafter referred to as an “engine”) has been known in which a pulley ratio is controlled by, using a motor, sliding a movable side of a drive pulley along an output shaft of the engine. In the controller, a target ratio is determined by searching a map based on a throttle opening and a vehicle speed, and the motor is driven such as to realize the target ratio. Furthermore, when the difference between the target ratio and the actual pulley ratio is larger, the duty ratio of the motor is made larger. At the same time, the duty is controlled according to whether an accelerator is on or off. The transmission controlled in the above manner is expected to operate smoothly meeting driving-condition-dependent requests for gear shifting.
For a vehicle carrying a continuously variable transmission, use of a control method in which plural drive modes are set for the vehicle and the shift characteristic is changed between drive modes has been under study. In such a vehicle, changing the shift characteristic at a time of a drive mode change causes, in a case where there is a large difference between a target engine speed and the actual engine speed, the transmission gear ratio to be changed sharply to cope with the large difference between the target and actual engine speeds. Such an arrangement causes a shift shock which may impair the riding comfort of the vehicle and affect the durability of the transmission.
Controlling the duty of a motor based on the difference between a target value and an actual value as in the controller described in JP-A No. 123351/1994 may be considered to solve the above problem. A shift shock caused by a change in shift characteristic resulting from a drive mode change, however, cannot be adequately reduced just by adjusting the duty of the motor according to a difference between pulley ratios. Hence, a further study has been considered necessary.